In C++,
A::B means B is an identifier within either namespace or class type A, A.B means B is a member of the struct, class, or union type an instance of which is referred to by the object or reference A, and A->B means B is a member of the struct, class, or union type an instance of which is referred to by the pointer A. (It's equivalent to (*A).B.)
In some other languages, all three cases are covered by a . only.
Note that in C++, member function don't have to be implemented (defined) within their class' definition. (If they are, they are implicitly inline.) They can be, and often are, implemented in separate implementation (.cpp) files. This has the advantage that not all users of a class need to recompile when you change an implementation of one of the class' member functions. So unless weight_data is a namespace name, void weight_data::rev_seq(string &seq) {...} is such a definition of a class member outside of its class.
